I have 2 dropdowns at my form, If I select the option of the index 1 of dropdown

If dropdown 2 is going to be read-only always, a read-only textfield might be a better option instead. FormAPI.setValue() can be used to set the appropriate value in the read-only textfield.

Elaborating on my respone above
You could get the appropriate index value from the dropdown1, based on the index you could set the exact value to the textfield using FormAPI.setValue().

I'm talking about index, for example: I've the dropdown1 {apple, banana, pear} and the dropdown2 {red, yellow, green}. If I select "apple" on dropdown1, dropdown2 should show "red". If I select "banana" on 1, 2 should show "yellow". If I select "pear", should show "green"... you got it?

My earlier posts are probably not clear. What you could do, is get the index of the selected item in your dropdown 1 using onchange event (your code for that will look something like - index = document.getElementById('dd_id').selectedIndex; Now, have an hidden field on your form with the values of the second dropdown. In your JS, you can get the value of this hidden field using FormAPI.getValue. Then, you could use the valueOf function to get the value at the corresponding index in the hidden field. This value can then be set as the value of your read-only text box (note: instead of using a read-only drop down, I am suggesting a text box).

jmenezes, curious why a read-only dropdown is needed when users are not going to be able to drop it down. Is it just for the look and feel? I'm sure 'Advanced Form Utilities' under forum shared components has features to implement the dynamic dropdown that you're looking for.

Thank you Ravi, I got it now and it worked.